1. Field of the Invention
The present invention relates, in general, to a radio communication system, and more particularly to a mobile communication system which establishes a radio communication connection with respect to signals sent from mobile wireless terminals. The mobile wireless terminals include, for example, cellular phones, portable telephones, and wireless LAN system which establish a communication connection for providing data transmission with respect to data sent from a computer or an office automation terminal through a wireless link, and more particularly of a type which includes a radio communication system having an independent cell structure containing forward and reverse links in which a service area for reverse links, each leading from the associated terminal to a radio base station, and a service area for forward links each leading from a radio base station to the associated terminal, are constructed using different cells.
2. Description of the Related Art
A conventional communication system which utilizes wireless links, and which is relatively economical is described in JP-A-63-87837.
The above-mentioned prior art system includes a local radio station covering a relatively wide area divided into a plurality of service areas (cells). The radio station performs a transmission operation using a relatively powerfull radio wave, which can be received by a terminal (e.g. a portable wireless terminal) in an arbitrary position within the plurality of cells. A plurality of relay radio stations each of which communicates with a terminal located in an associated cell using a weak radio wave within the above-mentioned individual cells in the area of coverage. In this connection, the local radio station is coupled with the relay radio stations through the channels.
The reverse signal, which has been transmitted from each wireless terminal using the weak radio wave, is received by the relay radio station within the associated cell and then is relayed to the local radio station through the associated channel. The forward signal which is to be transmitted from the local radio station to the terminal of interest is transmitted into the air using a transmitted power greater than that of the transmitted radio wave sent from each wireless terminal to be directly received by the terminal of interest.
The above-mentioned radio communication system is constructed in such a way that the reverse and forward links are made independent of each other by utilizing both the portable wireless terminals each having a function of transmitting the weak radio wave, and a plurality of radio equipment (relay radio stations) each of which is of a relatively weak power type, whereby the economical radio data communication is performed.
Undesirably, in the prior art shown in JP-A-63-87837, since the single local radio station is installed for the plurality of relay radio stations, each of which is installed in-house and operates to relay the very weak radio wave, the range of the service area is necessarily limited to the range in which the radio wave transmitted from the local radio station can be received by each terminal. Thus, the above-mentioned prior art is unsuitable with application to a communication system in which, as in case of cellular phones, the portable telephones, and the like, a large number of main base stations (local radio stations) are provided, and the service areas are repeatedly arranged, whereby the extensive service area is covered. In addition, the above-mentioned prior art is unsuitable for use with "the handover technique", which is essential to the cellular phone system, and the portable telephone system for example, and is utilized to hold the communication, when the terminal of interest is moved from one service area to another service area, without any interruption of the communication.
Incidentally, in the handover technique, disclosed, for example, in U.S. Pat. No. 5,179,571, there is a system in which the level of the received signal of the pilot signal sent from the radio station is monitored by each portable wireless terminal, and handover is performed utilizing the change in state of the received signal's level.
In the case where the handover method is intended to be applied to the communication system disclosed in JP-A-63-87837, the so-called "near-far problem" arises between the transmitted signal from the local radio station and the pilot signal transmitted by each weak-wave-relay radio station. That is, in the portable wireless terminal, which is located in the vicinity of the local radio station, since the level of the signal sent from the local radio station is too high, the pilot signal sent from the associated weak-wave-relay radio station can not be received. On the other hand, in the portable wireless terminal which is located far from the local radio station and in the vicinity of the associated relay radio station, since the level of the pilot signal sent from the associated weak-wave-relay radio station is too high, the signal sent from the local radio station can not be received. Therefore, it becomes difficult to realize the handover.